'Thermal pollution' in rivers not fully mediated by gravel augmentation

Although adding gravel to a river to replace lost sediments won't likely cool the whole river channel, it can create cool water refuges that protect fish from thermal pollution, according to a U.S. Forest Service Pacific Northwest Research Station study. The research—featured in the June 2011 issue of Science Findings, a monthly publication of the station—is among the first to explore the interplay between sub-surface water flow and temperature in large rivers and is helping to guide river restoration strategies in the Pacific Northwest.

In the study, which began in 2006, station research hydrologist Gordon Grant and Oregon State University colleagues Barbara Burkholder and Roy Haggerty examined the effect of subsurface water flow through riverbed sediment—a process known as "hyporheic flow"—on daily minimum and maximum water temperatures. The focus of their study was Oregon's Clackamas River, which, at the time, was undergoing intensive restoration planning efforts led by Portland General Electric (PGE) as part of the relicensing process for the river's hydroelectric system. The addition of gravel to the large river as part of these efforts—aimed primarily at reversing changes in river channel morphology that have resulted from sediment transport being interrupted by the dams—allowed the researchers to explore whether doing so had any measurable effect on reducing "thermal pollution," or unusually high water temperatures caused by human activities like dam operation, logging, and wastewater treatment.

"Previous work suggested that water emerging from gravel bars might actually be cooler than the surrounding water," said Grant.

The research team hypothesized that the continual cycling of subsurface water through the riverbed—during which cool nighttime water would travel through the gravel bar, exiting and mixing with the stream during the warmer daytime—would have a "buffering" effect that would keep the river's daily peak temperatures down, but not necessarily change the river's overall mean temperature. To explore their hypothesis, they mapped the locations of gravel bars along a 15-mile stretch of the river and documented the temperature of water cycling into and out of each of them.

They found 52 temperature differences within the stretch of the Clackamas, with temperatures at these locations from 1 to 4 degrees cooler than the main channel. The researchers were then able to link the cooler areas with specific gravel bar features and with specific times and locations within the Clackamas to create models that depicted the subsurface flow patterns—ultimately revealing that a very small percentage of the river's water actually passed through the gravel bars, making any overall effect on the mean temperature minute.

"Results showed a hundredth of a degree of temperature change through a single bar," said Grant. "Not much."

This finding suggests that gravel augmentation alone is not likely to have a significant temperature-mediating effect in large rivers. However, the work demonstrated that gravel augmentation may provide local habitat benefits to fish and small invertebrates by creating cool areas within rivers where they can seek refuge during hot weather.